Results from a novel carbonization process that pyrolyzes biomass in a sealed reactor (constant-volume carbonization) will be presented. The use of a sealed reactor results in a pressure increase as the heat treatment temperature is raised. Under these conditions, charcoals with near-theoretical limiting fixed-carbon yields are produced in a relatively short time period in comparison with conventional methods. Research is being conducted to determine the influence of each pyrolysis condition (peak pressure, reaction temperature, heating rate, reaction time) as well as the effect of fuel properties (biomass type and particle size) on product yields, and the char’s chemical and physical properties. Results will be presented which show the effects of pressure, temperature, heating rate, particle size, and processing time on product yields and properties. Prolonging the processing time (from 30 to 190 min) and raising the carbonization temperature (from 300 to 400 to 500°C) lead to greater devolatilization of the charcoal without sacrificing its valuable fixed-carbon content. The resultant charcoals are richer in fixed-carbon content and have lower volatile matter contents, compared to char produced at lower temperature. Increasing the pressure by addition of an inert gas prior to sealing the reactor (from 0 to 2.17MPa) did not impact proximate analysis results or product yields. Partial pressures of the tarry vapors, and not the total system pressure, appear to be accountable of the increase in fixed-carbon yields. SEM images of the charcoals showed that some of the chars had experienced a transient-plastic phase that transforms the char appearance from particulate to a single solid piece, this occurs when the char is exposed to temperatures and/or pressures above a certain level during the reaction. In contrast to flash-carbonization or traditional carbonization, small grains of feedstock can be used to produce high quality chars and with good yields via constant volume carbonization. This is a novel and important finding as it provides a pathway for the conversion of smaller pieces of biomass (agricultural waste, sawdust, etc.) into higher value charcoals and carbon materials.

Constant-volume carbonization of biomass

255th National Meeting (2018)

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